Method and medium for detecting the presence or absence of staphylococcus aureus in a test sample

ABSTRACT

A presence/absence test for  Staphylococcus aureus  ( S. aureus ) involves placing a first generation test sample in a solution that will clot in the presence of  S. aureus . The solution contains components that will selectively grow  S. aureus  and also contains clotting factors that will react with  S. aureus , if  S. aureus  is present in the sample, to clot the solution. Examples of specimen samples that can be tested include nasal swabs and lesion swabs, among others. The test can also be modified to detect the presence or absence of methicillin resistant  S. Aureus  (MRSA).

This application is a divisional of U.S. patent application Ser. No. 12/356,847 filed Jan. 21, 2009, which claims the benefit of U.S. Provisional Patent Application Ser. Nos. 61/062,144, filed Jan. 24, 2008; and 61/108,722, filed Oct. 27, 2008, all of which hereby incorporated by reference into the present application in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Information

The present method and test mixture relates to the detection of Staphylococcus aureus in a biological, environmental, or food sample, and more particularly to those methods and test mixtures utilizing reacting factors which the target microbe(s) can convert into a clot.

2. Background Information

Staphylococcus aureus (S. aureus) can be a virulent pathogen of animals and humans. Moreover, it can cause severe food poisoning by the production of a toxin. Diseases caused by S. aureus cover a very wide clinical spectrum, from simple skin infections to life threatening infections of the bones, heart, and organs. Of particular concern is the recognition that S. aureus infection is common after surgery. It is also associated with intravenous tubing and other implants.

The bacterium S. aureus may be transmitted between healthy individuals by skin to skin contact, or from a commonly shared item or a surface (e.g., tanning beds, gym equipment, food handling equipment, etc.) where the transfer may be made to a subsequent person who uses the shared item or touches the surface. Of great medical concern is the recognition that healthy people entering hospitals may “carry” S. aureus (e.g., on their skin, or in their noses, etc.) without any signs or symptoms that they do so. In the presence of favorable conditions (often found in but not limited to hospitals), the S. aureus can activate and cause serious infection. In addition, S. aureus can also be a source of food poisoning, often caused by a food handler contaminating the food product (e.g., meat, poultry, eggs, salads containing mayonnaise, bakery products, dairy products, etc.)

There are two categories of S. aureus based on an individual clone's susceptibility to the class of antibiotics that began with methicillin. These are methicillin susceptible S. aureus (MSSA), and methicillin resistant S. aureus (MRSA). Until only a few years ago, MRSA was most commonly found in hospitals. Now, many are also present in the noses, skin, etc. of people in the non-hospital community. Moreover, these MRSA are increasingly causing serious infections in the community. MRSA is particularly serious because very few antibiotics (e.g., vancomycin) have been shown to be uniformly effective against MRSA.

The Center for Disease Control and Prevention actively surveys for the development of methicillin resistant S. aureus. In 2000, the Society for Healthcare Epidemiology of America guidelines recommended contact isolation for patients with MRSA. In addition to the morbidity and mortality caused by MRSA, it has been estimated that each case of infection costs at least $23,000. Accordingly, many hospitals and nursing homes proactively sample patients for MRSA [Clany, M., Active Screening in High-Risk units is an effective and cost-avoidant method to reduce the rate of methicillin-resistant Staphylococcus aureus infection in the hospital, Infection Control and Hospital Epidemiology 27:1009-1017, 2006].

Meyer et al. (U.S. Pat. No. 4,035,238) describes the use of a broth for the detection of S. aureus that utilized mannitol as a source of carbon and DNA methyl green as an indicator. Neither of these chemicals are coagulase reactive substrates.

Rambach (U.S. Pat. No. 6,548,268) employs at least two chromogenic agents in an agar medium: 5-bromo-6-chloro-indoxyl-phosphate; and 5-bromo-4-chloro-3-indoxyl glucose in the presence of deferoxamine. An individual colony hydrolyzing these substrates will produce colors that will mix with each other and not be independent of one another.

A large number of classical culturing procedures are utilized to detect MSSA and MRSA from human, animal, food, etc. samples. They have in common a basic medium with chemical inhibitors such as 6-8% sodium chloride, potassium tellurite, and a variety of antibiotics. For example Stevens and Jones described the use of a trehalose-mannitol-phosphatase agar [Stevens, D L and Jones, C. Use of trehalose-mannitol-phosphatase agar to differentiate Staphylococcus epidermidis and Staphylococcus saprophyticus from other coagulase-negative staphylococci, J. of Clin. Microbiology 20:977-980, 1984]. The use of mannitol as a carbon source and salt as a selective agent into an agar known as mannitol-salt agar has been commonly used in clinical laboratories [Baird, R. M. and W. H. Lee, Media used in the detection and enumeration of Staphylococcus aureus, Int. J. Food Microbiology. 26:209-211, 1995]. Within the prior art of culturing, it is generally accepted procedure to perform coagulase tests utilizing samples of S. aureus that are isolated in a pure culture.

The procedure “S. aureus ID” [Bio Merieux, La Balme Les Grottes, France] uses an alpha-glucosidase substrate in agar to detect S. aureus. A single substrate is utilized. [Perry, J. D. et al., Evaluation of S. aureus ID, a new chromogenic agar medium for detection of Staphylococcus aureus, J. Clin. Microbiology 41:5695-5698, 2003]. A variant of this medium, which contains added antibiotics and sodium chloride, is designed to detect MRSA [Perry et al., Development and evaluation of a chromogenic agar medium for methicillin-resistant Staphylococcus aureus, J. of Clin. Micro. 42:4519-4523, 2004].

Selepak and Witebsky disclose a study evaluating the inoculum size and lot-to-lot variability of the tube coagulase test for S. aureus. Specimens were collected and isolates were generated from the bacterial colonies on agar plates. Tubes containing anticoagulated rabbit coagulase plasma were inoculated with a part of, or more than one, staphylococcal colony from the isolates. The tubes were incubated and examined for the presence of clot. According to Selepak and Witebsky, “with some isolates and some lots of coagulase plasma, even a single colony [from the isolate] may not provide enough inoculum for a positive coagulase test”. Furthermore, Selepak and Witebsky state that “[e]xpressed more quantitatively, at least 10⁸ organisms per ml should be used whenever possible for each coagulase tube test. Our data further suggest that S. aureus does not grow in coagulase plasma; therefore, the incubation of coagulase plasma for 18 to 24 h does not compensate for the use of small inoculum.” Thus, Selepak and Witebsky indicate that it is impractical, if not impossible, to detect the presence or absence of S. aureus in first generation biological specimen samples using a direct coagulase test.

[Selepak, S. T et al, “Inoculum Size and Lot-to-Lot Variation as Significant Variables in the Tube Coagulase Test for Staphylococcus aureus”, Journal of Clin. Microbiology, November 1985, p. 835-837]

It would, therefore, be desirable to provide a test mixture and a method that can rapidly detect S. aureus directly from a sample, one that does not require a skilled technician to perform the method, one that can be performed without the need to develop isolates from the specimen (i.e., one that can be performed on a “first generational” sample), and one that does not require a large concentration of S. aureus organisms to be accurate.

SUMMARY OF THE INVENTION

This invention relates to a method and test mixture for specific detection of S. aureus bacteria in a biological, environmental, or food sample. In the detection of S. aureus, a test mixture (which mixture may also be referred to as a “medium”) is utilized that includes coagulase substrates (sometimes referred to as “coagulase reacting factors”) that react specifically with coagulase produced by S. aureus to form a clot, admixed with constituents that facilitate the multiplication of S. aureus (also referred to as “growth promoting constituents”). Hence, the present method and test mixture utilize coagulase substrates that are activated by the coagulase produced by S. aureus, and the enzyme coagulase is specific to pathogenic staphylococci, as is disclosed in the Code of The Federal Register, Title 21, Chapter 1, Sub Part C, Sec. 866.2160 “Coagulase Plasma”. Inhibitors and antibiotics may be included to inhibit or otherwise negatively affect competing bacterial growth, but are not required. The untreated sample (e.g., collected from a nasal swab from a person, or off of a surface, etc.) is added to the test mixture, and the inoculated test sample is incubated. If S. aureus is present within the sample, the S. aureus will multiply within the test mixture and will produce coagulase that reacts with the coagulase substrates. The reaction between the coagulase produced by the S. aureus and the coagulase substrates within the test mixture will produce a detectable clot within the test mixture in a time period typically between two and twenty-four hours, positively indicating the presence of S. aureus.

Under the present method, the clot may be dissolved to release viable S. aureus into a liquid, which liquid can then be subjected to further analyses, including but not limited to: antibiotic susceptibility tests, molecular fingerprinting, genetic analysis, etc. As will be described below in greater detail, the antibiotic cefoxitin or an inducer of the mecA gene may be included within the test mixture to enable methicillin resistant S. aureus (MRSA) specific testing within the first generational sample, or the dissolved clot mixture may be tested to ascertain the presence or absence of MRSA therein.

The test mixture is preferably prepared in a form that facilitates handling, packaging, storing, etc., of the test mixture. A dry powder that can be hydrated into liquid form is a particularly preferable form for the test mixture, but the present invention is not limited to a powder form. The test mixture may assume a liquid form, or any other form (e.g., paste, gel, etc.), preferably one that can be hydrated for use.

The coagulase substrates within the test mixture may be provided within plasma, or may be provided by another substance that is operative to react with the coagulase produced by S. aureus to form a clot. Present testing indicates that rabbit plasma is a favorable source of a coagulase substrate. Other plasmas (e.g., pork plasma) may be used alternatively. Fibrinogen is another example of a source of a coagulase substrate. In those embodiments that utilize plasma as a source of a coagulase substrate, it may be preferable to add a non-plasma source of a coagulase substrate to the test mixture to ensure an adequate source of coagulase substrate within the test mixture. As an example, our testing indicates that combining fibrinogen and rabbit plasma within the test mixture is an effective means for ensuring a consistent, adequate source of coagulase substrates. An advantage of adding a material such as fibrinogen to the test mixture is that it increases the performance consistency of the test mixture, and makes the method less susceptible to variability that may occur with plasma.

The growth promoting constituents within the test mixture that facilitate the multiplication of and sustain S. aureus can be varied to suit the application. Those in the art will recognize that many different combinations of constituents, and varying relative amounts of the same constituents, can be used to provide the same functionality Growth promoting constituents include sources of nitrates and proteins, material operative to assist in the generation of nucleic acid synthesis, sources of energy for the S. aureus, sources of amino acid growth factor, and in some embodiments materials operable to help repair damaged target organisms. This list of growth promoting constituents does not represent all of the materials that can be beneficial within the test mixture, but does illustrate materials that are acceptable (e.g., vitamins, salts, minerals, inorganic moieties, etc.). The test mixture may include other constituents that benefit the performance of the test mixture.

In most applications of the present invention, it will be desirable to utilize a test mixture that includes the following: a) an effective amount of amino acids; b) an effective amount of nitrogen sources; c) an effective amount of salts; d) an effective amount of vitamins; and e) an effective amount of calcium. Those skilled in the art will recognize that natural sources of such amino acids can be used rather than pure sources. The natural sources (e.g. extract of whole organisms, such as yeast) may be in mixture form or in purified form. The natural mixtures can contain varying amounts of such amino acids and vitamins Those skilled in the art will further recognize that many different combinations of amino acids and vitamins can be used in present invention test mixture.

Those in the art will further recognize that carbon, nitrogen, trace elements, vitamins, amino acids and selective agents can be provided in many forms. Generally, it is preferred to have an amount of vitamins and amino acids within a predetermined range, but those in the art will recognize that the actual properties of each ingredient may be varied so that reduction in the amount of one ingredient can be compensated by an increase in the amount of another. This is particularly relevant when the essential amino acids, trace elements or vitamins of the microbes sought to be detected are known. Some ingredients may be provided in reduced amounts or deleted if they may be synthesized endogenously by the microorganism whose presence is to be determined Salts may be provided as a source of ions upon dissociation.

The test mixture may be packaged in a container (e.g., a test tube, a container with a flat bottom wall, etc.) that facilitates the testing process. If the medium is prepared in a form that can be hydrated, the mixture can be hydrated with sterile water or non-sterile water.

To detect the presence of MSSA or MRSA within a sample, the sample is obtained from a biological, environmental, or food specimen. A sample collected using a nasal swab is an example of a first generation sample that is particularly convenient to collect and test using the present invention. Once collected, the sample is inoculated into the test mixture.

The inoculated sample is incubated under conditions favorable to facilitate the multiplication of any S. aureus that may be present within the inoculated sample. In the case of a powdered test mixture hydrated with water, the incubation may be carried out at temperatures between about 20° C. to 42° C. The combination of sequential enzyme specificity, S. aureus enhancing growth factors, and antibiotic selectivity provides multiple hurdles which prevent the competing non-target bacteria from being detected within the test period; e.g. 24 hours or less.

The present invention test and method can be used in hospital admissions, routinely in intensive care units, in nursing homes, dialysis patients, people receiving home immunosuppressive therapy, and the like. It can also be used in environmental settings (e.g., gyms, tanning salons, restaurants, etc.) where the bacteria S. aureus may be transferred from a human carrier and it can be used to test various different foods for S. aureus contamination. It will be appreciated that a substantial benefit of the present method and mixture is that they may be performed/used without the need for expensive equipment or skilled medical technologists. Another substantial benefit of the present method/mixture is that it is operable with a relatively small amount of S. aureus within the test sample; e.g., the present method/mixture has detected S. aureus in samples having concentrations of S. aureus as low as 100 CFU/ml.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of the invention will become more readily apparent from the following detailed description of several embodiments of the invention when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a side elevational view of a test tube containing a powder culturing mixture which is formulated to detect the presence or absence of S. aureus in a first generation biological sample of a nasal swab;

FIG. 2 is a side elevational view of the test tube of FIG. 1, but showing the culturing mixture having been hydrated by water;

FIG. 3 is a side elevational view of the test tube FIG. 2 and showing a cotton swab inserted into the test tube to deposit a first generation biological specimen nasal swab in the medium;

FIG. 4 is a side elevational view of the test tube of FIG. 3 after the specimen has been deposited and cultured in the medium for a period of time and indicating the absence of S. aureus in the specimen; and

FIG. 5 is a side elevational view similar to FIG. 4 but showing the test tube medium after the culturing period and indicating the presence of S. aureus in the specimen.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

FIG. 1 is a side elevational view of a test tube denoted by the numeral 2 which preferably has a flat bottom 4 and a top closure 3, and which contains a dry powdered test mixture 1 which is formed in accordance with this invention for detecting the presence or absence of S. aureus in a sample; e.g., a first generational biological sample. The tube 2 is also provided with a reference line 5 which indicates the amount of water to be added to the tube 2 in order to properly hydrate the powdered mixture 1 for specimen sample testing.

Acceptable hydrated test mixtures can be made using the following constituents in the ranges indicated, to create 15 ml of test mixture:

Specific Examples of Range of Constituent Constituent Quantities per Quantities per 15 ml Test Constituent 15 ml of Test Mixture Mizture Nitrate Broth 7.5 ml 1.0 ml-9.0 ml  Water 7.5 ml 1.0 ml-9.0 ml  Uracil 10.0 mg 1.0 mg-20.0 mg Sodium Pyruvate 10.0 mg 1.0 mg-20.0 mg L-glutamine 20.0 mg 5.0 mg-40.0 mg Sodium Sulfite 1.0 mg 0.5 mg-2.0 mg  Rabbit Plasma 100.0 mg 15.0 mg-500.0 mg Fibrinogen 100.0 mg 15.0 mg-500.0 mg

The specific example of the constituent quantities per 15 ml of test mixture provided above represents a particular test mixture formulation that was tested and found to perform satisfactorily. This specific example does not represent all test mixture formulations, and the present invention is not limited thereto. As stated above, those in the art will recognize that many different combinations of constituents, and varying relative amounts of the same, can be used to provide the same functionality. Hence, the present method and mixture contemplates that a number of different constituent formulations can be made within the aforesaid ranges.

As noted in FIG. 2, the powdered mixture 1 is properly hydrated by the addition of water, preferably distilled water, to form a hydrated test mixture 6 into which the sample (e.g., carried on a nasal swab) is deposited.

First generational test samples can be collected by a variety of different techniques; e.g., a human sample can be collected by wiping a swab within the nose of a subject. Nasal swabs are a particularly convenient way of collecting a test sample, but they are not the only collection method; e.g., test samples can be collected from throat swabs, skin lesions, undamaged skin, etc. First generational environmental samples can be collected by various known methods; e.g., wiping or swabbing a surface using a dry or wet wipe/swab. Likewise, first generational food samples can be collected from the food itself, or wiping food residue from surfaces in contact with the food, etc. Once the sample is collected, it can be deposited in the hydrated test mixture 6; e.g., using the same cotton swab 8 which has been used to gather the first generation sample from the source thereof. Once the specimen sample is deposited in the test mixture 6, it is incubated within the test mixture for a period of time typically less than twenty-four hours. The incubation may occur at any temperature that is acceptable under the circumstances. After the inoculation period, the container (e.g., test tube 2) holding the inoculated test mixture can be inspected for the presence of a clot; e.g., the test tube 2 can be tilted to one side as shown in FIGS. 4 and 5 to see if the meniscus 10 of the text mixture will move or whether a clot keeps the test mixture below a reference line 5. The presence of a clot indicates that S. aureus is present in the test sample, and the absence of a clot in the inoculated test mixture indicates that S. aureus is not present in the test mixture 6, as shown in FIG. 4. In some instances, the entire inoculated test mixture will clot, and in others some liquid will remain in the container with the clot. Approximately 80% of the present tests performed using first generation nasal samples clotted within six hours when S. aureus is present in the first generation test sample.

To determine the effectiveness of the present method and mixture, a control study was performed involving sixty (60) control samples titred to contain varying amounts of MSSA, and sixty (60) control samples containing varying amounts of MRSA. Standard clones of MSSA and MRSA were grown in trypticase soy broth (TSB), and were diluted by log 10 increments. The present invention test mixture was inoculated with a set amount (0.1 ml) of each the control samples. A first set of the inoculated test mixtures were incubated at 35° C., and second set of the inoculated test mixtures were incubated at 23° C. Of the sixty control test samples, all were positive for S. aureus in five hours, forty-nine (49) were positive in four hours; thirty-six (36) were positive in three hours, and twenty-four (24) in two hours. Data detailing the relationship between the concentration of the inoculum, and incubation temperature was as follows:

S. aureus CFU/ml Clot at 35° C. Clot at 23° C. 7 log 10 2.0 h 3.0 h 6 log 10 3.0 h 3.0 h 5 log 10 4.0 h 4.0 h 4 log 10 6.0 h 7.0 h 3 log 10 10.0 h  11.5 h  2 log 10 15.0 h  21.0 h  The concentration of S. aureus within the clots were all at least 5 log 10.

In addition to the above described control study, a clinical study was performed using fifty samples. The samples were taken from a medical intensive care unit by culturing patient nares. The patients were not identified, nor were the results of any “standard” culture available (FDA protocol). The samples were plated on mannitol salt agar (MSA) using swabs. After plating the samples on MSA, the swabs were used to inoculate the test mixture. Clotting was observed for each hour for twenty-four hours. There were no false positives.

Number of Number of Number of samples samples detected samples detected detected positive for positive for positive for identified bacteria by identified bacteria identified both MSA method and by present bacteria by Bacteria present method method alone MSA method alone MSSA 9 2 — MRSA 13 2 1 When a clot was observed, a portion of the clot was removed and dissolved. A quantitative count of CFU/ml was performed from the dissolved clot material.

In some embodiments, the present method/mixture may include means to distinguish between MSSA and MRSA. For example, cefoxitin in a concentration of about 10-100 mg/ml or another MecA gene inhibitor can be included in the test mixture. Any MSSA present within the test sample will be killed, but MRSA will not. Thus, if a clot does form, the S. aureus in the test sample will have been shown to be MRSA. If a clot forms and confirms the presence of MRSA, the clot can then be dissolved in order to perform further analyses of the S. aureus bacterium detected.

It will be appreciated that the test of this invention is significantly simpler to perform than the standard tests which are currently in use, as typified by the coagulase plasma procedure suggested by Remel Products, Thermo Fisher Scientific, Lenexa, Kans. , U.S.A. The Remel procedure, which is approved by the FDA and appears in the Code of the Federal Register as an exempt test, requires a two step test for S. aureus wherein microbe colonies from the specimen are first grown in an agar medium and screened for suspected S. aureus colonies using a gram stain and catalase slide test before proceeding to a second coagulase test step. There are complications relating to this type of coagulase test, namely: 1) colonies for coagulase testing must not be picked from media containing high concentrations of salt as false positive results may occur; 2) in the first step slide test procedure, the organism/saline suspension must be observed for auto-agglutination prior to the addition of the coagulase plasma to prevent a false positive test reading; and 3) false negative coagulase reactions may occur if the test culture is older that 18-24 hours, of if there is scant growth.

While the invention has been described with respect to preferred embodiments, those skilled in the art will readily appreciate that various changes and/or modifications can be made to the invention without departing from the spirit or scope of the invention as defined by the appended claims. 

1. A liquid test mixture for use in detecting the presence or absence of Methicillin Resistant Staphylococcus Aureus (“MRSA”) in a first generation biological specimen, the test mixture comprising: a) an effective amount of amino acids; b) an effective amount of nitrogen sources; c) an effective amount of salts; d) an effective amount of vitamins; e) an effective amount of calcium; f) an effective amount of coagulase substrates which will clot the test mixture in the presence of S. aureus in the test sample; and h) an amount of a MecA gene inducer effective to kill any Methicillin Susceptible Staphylococcus Aureus (“MSSA”) in the specimen.
 2. The liquid test mixture of claim 1 wherein the MecA gene inducer is cefoxitin. 